In addition to recombination, retroviruses also interact with one another via complementation, which involves the copackaging of viral proteins and/or RNAs. The goals of this project are to gain further insights into virus interactions by studying the mechanisms of virus assembly, RNA packaging, and replication. We have dissected a motif that is important for virus assembly and located at the C-terminal region of murine leukemia virus (MLV) capsid (CA). Mutations in this motif often cause a mutant phenotype that has virus assembly defects. These mutants produce virions that have aberrant virion morphology and package both viral RNA and ribosomal RNA. From data generated in our mutational analyses, we hypothesize that the sequences in this motif form an a-helix; maintenance of the helical structure and the phase of the helix are critical to its function. We are performing experiments to further investigate this motif to gain insight into the functional domains of CA in virus assembly. MLV and spleen necrosis virus (SNV) have a nonreciprocal relationship in RNA packaging; SNV proteins package both MLV and SNV RNA whereas MLV proteins only recognize MLV RNA. We have utilized this relationship to define the regions in the nucleocapsid (NC) domain of the Gag polyprotein and portions of the RNA in the packaging signals that are important for this specific recognition. We are further dissecting elements important in the protein-RNA interaction that define the RNA packaging specificity.